Optical device for measuring the speed or length of a moved surface

ABSTRACT

The invention relates to a device for measuring the speed or length of a moved surface, where a measuring light beam from a laser is used. The laser light beam reflected from the surface to be measured is frequency-shifted by the Doppler effect so that the overlaying of the outgoing light beam and the incoming one results in a beat which is a measure of the speed of the surface. Before the speed can be determined, digital signals occurring in large numbers must be processed in an evaluation system. The evaluation system is characterized in that there is downstream of a counter intended for detecting the incoming signals and having a high counting speed an intermediate store which is connected to an adding mechanism for pulse-wise monitoring of the counter contents. A control mechanism handles coordination of the counter, the intermediate store and the adding mechanism, upstream of which is a pulse generator. Counting results are added in the adding mechanism and occur at a substantially lower speed than the individual measuring signals.

The invention relates to an optical device for measuring the speed orlength of a moved surface, where a measuring light beam from a laser isdirected towards the surface and the Doppler-shifted dispersed lightreflected from the surface and other light, for example laser lightwhich is not Doppler-shifted or another Doppler-shifted laser light, areoverlaid to produce a beat whereof the frequency forms a measure of thespeed of the surface and is received and evaluated in an evaluationsystem in the form of continuously occurring digital signals.

In the development of the evaluation system, the fact that the lengthinformation obtained by the measuring process is based on a quantizationin the order of magnitude of 10 μm must be taken into account. Becauseof the speed of the material measured, which may be high, and thefrequency offset used, the measured frequencies may be up to 10 MHz. Forthe measuring process to be accurate, it is necessary for the evaluationsystem to detect and evaluate all the length increments.

When using the hitherto conventional analog evaluation systems havingso-called tracking filters, false measurements are often produced,particularly if the useful signal cannot be clearly separated in theevaluation system from the noise components.

In a further known evaluation system, the measured frequencies aremeasured in a measuring cycle and evaluated by integration in asubsequent evaluation cycle. It is true that here only an individualrapid counter of low counting capacity and a downstream adding mechanismof low speed are used so that the expenditure for equipment remains onan acceptable scale; however, with this solution it is not possible tocount and add at the same time, so that individual length increments arenot detected, so that falsifications in the result are caused by thealternating measuring and evaluation cycles.

With a maximum length of measured material of 100 km, evaluation mayalso be performed by a synchronous 35-bit incremental/decrementalcounter having a maximum counting frequency of 10 MHz. However, thissolution would necessitate a considerable expenditure for hardware.

It is thus the object to provide an evaluation system by means of whichthe signals, namely the measured frequencies, are received and evaluatedcontinuously without loss of information, without a disproportionatelyhigh expenditure being necessary for this.

In accordance with the invention, this object is achieved in that, in anoptical device of the type mentioned at the outset, there is downstreamof a counter intended for detecting the signals and having a highcounting speed an intermediate store which is connected to an addingmechanism for pulse-wise transmission of the counter contents, in that acontrol mechanism for sequence control is connected to the counter, theintermediate store and the adding mechanism, and in that a pulsegenerator is upstream of the control mechanism.

The main advantage of this evaluation system according to the inventionconsists in the fact that it receives and processes the signalsoccurring continuously from the optical system, that is to say themeasured frequency values, continuously, so that thus all the lengthincrements are measured continuously at any speed of measured materialand a correspondingly high accuracy of measurement results.

For the simultaneous rapid counting of the signals, a correspondinglyrapid counter is provided which can, however, have a low countingcapacity and thus requires only a low expenditure by comparison with theprior art.

The signals which have been counted by the counter within apredetermined sampling time ar transferred, once the sampling time hasexpired, to the store, which passes the signals on to the addingmechanism, where they are added, or subtracted as appropriate, to theprevious result. The essential point is that there is available for thistransfer and addition a period of time which corresponds at most to thesampling time, that is to say the pulse time. This time is sufficientfor this function to be carried out by a program-controlledmicroprocessor. The sequence control, namely the coordination of thecounter, the intermediate store and the adding mechanism, is handled bythe control mechanism, to which a sampling time for the control of theabove-mentioned components is predetermined by the upstream pulsegenerator.

This construction allows available integrated counter modules to be usedand the adding procedure to be carried out in a microprocessor system.Here, counting mechanisms of virtually any size in relatively slowcircuit portions, such as a microprocessor, can be used. Incidently,fluctuations in the sampling time do not influence the accuracy ofmeasurement, since no length increments are lost.

In accordance with a further development according to the invention, itis provided for the inlet of the evaluation system to be coupled to thecontrol mechanism such that the pulse is synchronized to the inletfrequency. This prevents metastable states with the inlet frequency. Thesynchronization of the pulse and inlet frequency is restricted tomatching of the signal edges.

The invention is explained in more detail below with reference to anexample embodiment.

The drawing diagrammatically illustrates a circuit of a portion of theevaluation system which handles counting and adding of the signalsemitted continuously by the optical system.

At the inlet of the evaluation system, at which the incoming signalsF_(in), namely the measured frequencies, are accepted, an inlet line 1branches into a line 2 and a line 3. The line 2 leads to the inlet sideof a counter 4 which operates rapidly but with a low counting capacity,in the present embodiment a 16-bit counter. At the outlet side, thecounting stages CT0 to CT15 are connected by way of lines 5 to storestages D0 to D15 at the inlet of an intermediate store 6. The outlets ofthe intermediate store 6 are connected by way of lines 8 to the inletstages P0 to P15 of an adding mechanism 9. The separation line at 7 isintended to indicate that the circuit portion to the left of this linecan be in the form of an integrated circuit and the portion to the rightof this line can be in the form of a program-controlled microprocessor.At the outlet of the adding mechanism, the measured values leave thisportion of the evaluation system as a continuously incremented item oflength data to a maximum of 2³⁶ ×10 μm. The lines 12, which lead backfrom the outlet side of the adding mechanism to a second inlet thereof,are a symbol of the continuous addition of the signals accepted from theintermediate store 6 to the addition results present.

Arranged in the left-hand portion of the circuit as a pulse generator 13is a generator which produces the operating pulse, namely a samplingtime of, in the present case, 100 μs, for the sequence control. Thepulse frequency generated by the pulse generator 13 of, in the presentcase, 10 kHz, is passed by way of a line 14 to a control mechanism 15which handles the sequence control and for this purpose is connected byway of a line 16 to the counter 4, by way of a line 17 to theintermediate store 6 and by way of a line 18 to the counting mechanism9.

From the inlet of the circuit, the branch line 2 leads to the controlmechanism 15, so that the inlet signal F_(in) can be synchronized to thefrequency of the pulse generator 13 in the control mechanism 15.

When the optical device is in operation, the inlet signals F_(in)received continuously by the circuit by way of the inlet on the lines 1and 2, representing the measured frequencies and lying within the rangeof 10 kHz to 10 MHz, are counted continuously in the counter 4. After asampling time predetermined by the pulse generator 13 has expired, thecounter state is transferred, with a delay of 1/(2F_(in)), to theintermediate store 6 and at the same time the counter four is set tozero. However, the counter 4 can also be reset at another point in time,but must be in good time for the remaining counting capacity to besufficient for a sampling time.

Then, the stored value is transferred from the intermediate store 6 byway of the lines 8 to the adding mechanism 9 and is added there, orsubtracted as appropriate, to the old result. For transfer and additiona period of time of at most the sampling time, which is here 100 μs, isavailable. This time is sufficient for the functioning of the addingmechanism 9 in the form of a program-controlled microprocessor.Triggering is effected by an external program interrupt.

The total count of all the length increments which have occurred up tothat point is thus available within the pulse of the sampling time of100 μs at the outlet of the evaluation system, by way of the lines 11.In conclusion, it is stressed once again that by means of thisevaluation system, which is to be understood merely as an exampleembodiment, all the inlet signals F_(in) are continuously received andevaluated without interruption. In relation to the continuous evaluationwithout loss of work, the expenditure for the device is relatively low.

I claim:
 1. A device for measuring the speed or length of a movedsurface using a light beam from a laser directed towards the surface,receiving a Doppler-shifted dispersed light reflected from the surfaceand producing a beat signal in the form of a continuously occurringdigital signal, wherein the frequency of the beat signal forms a measureof the speed of the surface, the device comprising:a counter forcounting the beat signal; an intermediate store, coupled to an output ofthe counter, for receiving a count of the beat signal; an addingcircuit, coupled to an output of the intermediate store, for adding thecount of the beat signal to a first result; a pulse generator forgenerating an operating pulse; and a control mechanism, responsive tothe operating pulse, for controlling a sequence of the counter countingthe beat signal, the intermediate store receiving the count and theadding circuit for adding the count to the first result.
 2. A deviceaccording to claim 1, wherein the beat signal is coupled to the controlmechanism and wherein the control mechanism synchronizes the beat signaland the operating pulse.